The present invention relates to a transparent coloring composition and a color filter. More particularly, the present invention relates to a transparent coloring composition containing an organic-inorganic composite pigment exhibiting not only a sharp particle size distribution but also an excellent photofatigue resistance as well as excellent spectral properties, a photosensitive transparent coloring composition obtained from the coloring composition, and a color filter obtained from the transparent coloring composition or the photosensitive transparent coloring composition.
Pigments have been primarily used for hiding a color tone of a substrate material and for changing the color to that of the pigments by coating the surface of the substrate material therewith together with a vehicle. The hiding power of the pigments is exhibited as a sum of surface reflection of incident light, and absorption and scattering of the light by the pigments (refer to “Handbook for Technology of Color Materials” edited by Institute of Color Materials, published by Asakura Shoten (1989), p. 240). In addition, the hiding power is influenced by a size, a dispersion degree and a volume concentration of the pigment particles, a refractive index of the vehicle, etc. On the other hand, the pigments having excellent photofatigue resistance, heat resistance and chemical resistance, have also been used as transparent colorants in extensive applications by further imparting a transparency thereto in addition to these excellent properties. One example of the applications is a color filter for liquid crystal displays. With remarkable progress of liquid crystal techniques, liquid crystal displays have been extensively used in personal computers as well as portable telephones, digital cameras, etc., and these liquid crystal displays are increasingly shifted to full color types. The liquid crystal devices are operated on the basis of such a principle that a liquid crystal enclosed between a pair of transparent electrodes is reversibly changed in orientation by applying an electric field to the electrodes, thereby producing shading due to change in transmittance of polarized light passed therethrough. The full coloration of the liquid crystal displays is achieved by providing regular pattern layers composed of three primary colors including red, blue and green, or cyan, yellow and magenta in respective fine regions on the transparent electrodes. The thus obtained patterned optical device constitutes a color filter, and have been required to be further improved in optical properties such as color properties, transmittance and contrast.
Also, a color filter for image pickup tubes is an optical device that is inevitably required to digitize full color images, and has been required to be considerably minimized in order to reproduce full color images having a high definition or resolution.
Typical examples of the method for production of a color filter for liquid crystal devices may include a pigment dispersion method, a dyeing method, a printing method, an electrodeposition method, etc. The dyeing method tends to be complicated in the production process and deteriorated in photofatigue resistance of the obtained product, and is, therefore, not frequently used. The printing method is still insufficient in accuracy irrespective of excellent productivity and low costs. The electrodeposition method has such a problem that the obtained product tends to be deteriorated in light transmittance due to formation of double ITO films required as transparent electrodes. On the other hand, the pigment dispersion method is simplified in production process and provides a color filter having an excellent photofatigue resistance, and, therefore, has been presently used as a main technique for production of the color filter (CMC, “2001 Markets of Constituting Materials for Liquid Crystal Displays”, (2001), p. 115).
In order to produce a pigment dispersion-type color filter, there have been proposed methods of patterning a coating film made of a composition prepared by dispersing fine pigment particles having specific particle sizes in a photosensitive resin, by photolithography (Japanese Patent Application Laid-open (KOKAI) Nos. 60-129707(1985), 60-129739(1985), 9-197118(1997), 2001-214077 and 2002-328215).
In addition, there has been proposed a method of patterning a resin layer in which a soluble pigment precursor is dissolved in a molecular state, and then converting the soluble pigment precursor into an insoluble pigment by deprotection reaction using a chemical, thermal or photolytic method to form a transparent colored pattern (Japanese Patent Application Laid-open (KOKAI) No. 8-6242(1996)).
As the photosensitive resin, there have been proposed water-soluble polymers substituted with photo-crosslinking group (Japanese Patent Publication (KOKOKU) No. 3-49422(1991)), radically photo-polymerizable resins composition (Japanese Patent Application Laid-open (KOKAI) Nos. 10-260310(1998) and 2000-47022), chemically amplified-type photoresists based on a catalytic reaction initiated by an acid produced from a photo-acid generator (Japanese Patent Application Laid-open (KOKAI) Nos. 7-84111(1995), 9-203806(1997), 10-260532(1998) and 10-39127(1998)), or the like.
Further, there have been proposed methods of subjecting an alkali-developable positive photoresist layer formed on an alkali-soluble resin layer in which fine pigment particles are dispersed, to exposure to light and to alkali development, and then etching and removing portions of the pigment-containing layer which are uncovered with the thus alkali-developed resist pattern (Japanese Patent Application Laid-open (KOKAI) Nos. 2000-53908 and 11-153706(1999)).
Meanwhile, the contrast of displays strongly depends upon depolarization of a color filter used therein. The depolarization further depends upon a scattering cross section of pigment contained in a pigment-dispersed film (Mure, Araizaki, Deguchi and Fujii, “SumitomoKagaku, 1998-II”, p. 11). Since the scattering cross section is determined by a refractive index ratio between the pigment and binder resin, and a particle size of the pigment, the depolarization may be improved by reducing the particle size of the pigment and the scattering cross section. Therefore, optical properties of the color filter be improved by minimizing the particle size of the pigment particles contained in the colored film. However, even though a primary particle size of the pigment particles is sufficiently small, if the pigment particles are secondarily agglomerated, the pigment particles finally have a large particle size, thereby failing to improve the optical properties of the color filter. Namely, the small primary particle size is a necessary condition, but is not a sufficient condition.
On the other hand, in order to obtain an organic-inorganic composite pigment composed of inorganic pigment particles and an organic pigment adhered onto the surface thereof, there have been proposed techniques for synthesizing or precipitating the organic pigment in the presence of inorganic pigment, techniques for adhering the organic pigment onto the surface of inorganic pigment by a mechanically mixing and milling treatment, techniques for dissolving the organic pigment in a solvent, dispersing inorganic pigment in the obtained solution and adding a poor solvent for the organic pigment to the resultant dispersion, etc., (Japanese Patent Application Laid-open (KOKAI) Nos. 4-132770(1992), 7-310027(1995), 10-88032(1998), 11-181329(1999), 2002-356625 and 10-183021(1998)).
Also, in order to obtain a scratch-resistance color filter, there has been proposed a method of adding silica particles surface-treated with an organosilane containing hydrolyzable group(s) to a coloring composition for color filter (Japanese Patent Application Laid-open (KOKAI) No. 10-268124(1998)).
Color filters produced by the pigment dispersion method have been extensively used in applications such as mainly liquid crystal displays because of merits such as excellent photofatigue resistance and simplified production process. The color filters have been required to be further improved in spectral properties such as color reproducibility, transmittance and contrast. In addition, in order to obtain a color filter for image pickup tubes, it has been demanded to provide a coloring composition which is not adversely influenced by the particle size of pigment used therein even when the pigment is formed into ultrafine particles due to the increased number of pixel. To meet the above requirements, it has been required to provide a coloring composition and a color filter in which a pigment having a particle size as fine as possible and a narrow particle size distribution is dispersed.
In order to pulverize the pigment into fine particles, there has been most frequently used a mechanical pulverization method. For example, in Japanese Patent Application Laid-open (KOKAI) Nos. 60-129707(1985), 60-129739(1985) and 9-197118(1997), there are described color filters using a pigment having a specified particle size. However, since the pigment is mechanically pulverized into fine particles, the resultant pigment particles have a non-uniform particle shape and fails to show a narrow particle size distribution.
In addition, in Japanese Patent Application Laid-open (KOKAI) Nos. 2001-214077 and 2002-328215, there are described methods for forming a pigment into fine particles by pulverizing the pigment together with a milling medium such as common salt, and then removing the milling medium by water-washing. However, the obtained particles may fail to have uniform particle shape and particle size distribution.
Further, in the technique described in Japanese Patent Application Laid-open (KOKAI) No. 8-6242(1996), there can be produced fine pigment primary particles. However, in this method, a long heating time is required for formation of the pigment in a polymer solid film (refer to Ichimura, “Functional Materials”, Vol. 23, No. 3, p. 25(2003)), and a color tone thereof may be unstable.
In Japanese Patent Application Laid-open (KOKAI) Nos. 4-132770(1992) and 7-310027(1995), there are described organic-inorganic composite pigments obtained by synthesizing and precipitating organic pigment by conducting azo-coupling reaction in the presence of inorganic pigment having a refractive index of not less than 2.0. However, these techniques aim at attaining a hiding effect by the pigment, but may fail to obtain an organic-inorganic composite pigment having a high transparency.
In Japanese Patent Application Laid-open (KOKAI) No. 10-88032(1998), there is described the technique for mechanically mixing and milling inorganic pigment and organic pigment while both are kept in a solid state. However, in the specified particle size range of the composite pigment composition, although it is possible to hide the pigment, the obtained composite pigment may fail to show a high transparency.
In Japanese Patent Application Laid-open (KOKAI) No. 11-181329(1999), there is described the technique of dispersing organic pigment in cyclic silicone in which a modified silicone is dissolved, adding inorganic pigment to the obtained dispersion, and evaporating and removing the cyclic silicone from the dispersion. However, in the specified particle size range of the inorganic particles which aims to attain a hiding effect, although it is possible to hide the inorganic pigment, the obtained composite pigment may fail to show a high transparency.
In Japanese Patent Application Laid-open (KOKAI) No. 2002-356625, there is described the technique for producing organic-inorganic composite particles by modifying the surface of white inorganic particles with organic residue and then adhering organic pigment onto the thus modified surface of the white inorganic particles. However, in this KOKAI, there is not described a coloring composition having a high transparency such as those used for color filters.
In Japanese Patent Application Laid-open (KOKAI) No. 10-183021(1998), there is described the transparent coloring composition containing organic-inorganic composite pigment treated with a silylating agent. However, there is no description concerning the particle size required for ensuring a transparency of the organic-inorganic composite pigment.
In Japanese Patent Application Laid-open (KOKAI) No. 10-268124(1998), there is described the technique of adding silica particles surface-treated with an organosilane containing hydrolyzable group(s) to a coloring composition for color filter. However, since the coloring pigment and silica particles are separately added to the coloring composition, the pigment particles tends to be agglomerated, so that it may be difficult to obtain a coloring composition for color filter having a high transparency.
As a result of the present inventors' earnest studies for solving the above problems, it has been found that when the surface of fine white inorganic particles is treated with an organic surface modifier and then the resultant surface-treated fine white inorganic particles are mixed and milled with an organic pigment, it is possible to obtain an organic-inorganic composite pigment containing primary particles having an average particle size of 1 to 100 nm and exhibiting a high transparency. The present invention has been attained on the basis of this finding.